1. Technical Field
The invention relates to a category of lasers including vertical cavity lasers, of design known as surface emitting lasers. Most important, such devices appear to satisfy the desire for integratable lasers--lasers to serve in Opto Electronic Integrated Circuits as well as in all-optic circuits. Contemplated integrated circuits may include electronics--generally semiconductor electronics--serving both for operation of the lasers and for other purposes.
2. Description of the Prior Art
Virtually from inception, the emergence of the laser raised expectations of widespread use in integrated circuits--both ancillary to electronic circuitry and in all-photonic circuitry. The development of the electrically pumped pn junction laser promised to satisfy the desire. Nevertheless, commercially expedient integrated lasers are not a reality. While there have been a variety of obstacles, I.sup.2 r loss coupled with high lasing threshold values are central. For specialized purposes, cooled circuitry might suffice; for general use a more economical approach is needed.
Worldwide effort has addressed the very promising Surface Emitting Laser aka Vertical Cavity Laser, and the consensus is that this approach points the way to commercially feasible OEICs. It is likely prevalent SELs will be based on active regions containing one or more Quantum Wells although active regions based on bulk material are not to be discounted. References tracing introduction and recent development are: Y. Arakawa and A. Yariv, "IEEE J. Quantum Electron.", QE-21, 1666 (1985); Gourley et al, "Applied Physics Letters", 49 (9), 489 (1986) and J. L. Jewell et al, "Optical Engineering", 29, 210-214 (1990).
Effort at this time is directed to an SEL structure consisting of a p-n junction active region in which photons are generated responsive to pumping current--an active region which in earliest work is based on "bulk", likely homogeneous composition, and which, in later work, makes use of Quantum Wells or of superlattice structure. The number of quantum wells, more generally the thickness of the active photon-producing material layer, inescapably dictates lasing threshold. Desired reduction in I.sup.2 r heating has led to a decreasing number of QWs, culminating in the 2- or the 1-quantum well structure of U.S. Pat. No. 4,999,842 dated Mar. 12, 1991. Most effective cavitation is due to the very high reflectivity resulting from use of Distributed Bragg Reflectors (with reflectivities well over 99%, e.g. for 20+pair mirror structures on both sides of the active region). See U.S. Pat. No. 4,999,842 describing a structure having a laser emission threshold at 7 microwatts/.mu.m.sup.2 for DBRs of 24-pair, 1/4 wavelength (1/4.lambda.) layers of GaAs and AlAs, embracing an active region based on an 80 .ANG. active layer of In.sub.0.2 Ga.sub.0.8 As (1/32 wavelength quantum well) emitting at 980 nm.
While the described work has resulted in acceptable lasing threshold values in the active material itself, heating due to high series resistance in the SEL pump circuit--a circuit including a p-type DBR, the active region, and an n-type DBR--continues to be a problem. Total power efficiency and maximum power obtainable from SELs continues to be low compared to that obtainable from edge-emitting structures (about 5% efficiency and 1 mW power output for SELs vs. 30% and 100 mW) for edge emitting structures. Origin of the problem is largely the p-type DBR--of the high series resistance due to low hole mobility and the high optical absorption resulting from increased p-type doping introduced to reduce resistance. Extensive effort directed to this problem has resulted in optimization of layer-to-layer interfaces in the mirror (allowing Continuous Wave room temperature operation without heat sinks--but only at the indicated performance level). Other effort has taken the form of high p-carrier doping levels either throughout the DBR or at the lowermost level (Y. J. Yang et al, App. Phys. Letters, vol. 58, pp. 1780-1782 (April 1991 ) as well as reduced number of Bragg pairs (by partial, or even complete, substitution of Bragg layers by silver). Both approaches result in associated optical absorption to lower the differential quantum efficiency of the cavity. While the trade-off (of lower resistance for lower quantum efficiency) is a useful design consideration, the overall problem remains unsolved.